The invention relates to an iterative Multiple-Input and Multiple-Output (MIMO) receiver and a corresponding method for receiving and blockwise processing of symbols in a MIMO transmission system. In particular the invention relates to a wideband MIMO receiver comprising at least two receive antennas, wherein the receiver comprises an adaptive equalizer, a MIMO symbol detector, a decoder, a first feedback path for feeding back soft information from the decoder to the MIMO symbol detector and a second feedback path from the decoder to the adaptive equalizer for feeding back soft information about estimated transmit symbols to mitigate intersymbol interference (ISI) and inter-antenna interference (IAI).
Future mobile communication standards like LTE-Advanced make use of MIMO techniques in order to fulfill the increasing demand on high spectral efficiency. Hereby, spatially-multiplexed data streams are transmitted and received by multiple antennas, i.e. transmit symbols are radiated simultaneously on the same radio resources via at least two transmit antennas and a receiver receives the radiated transmit symbols via a plurality of at least two receive antennas. In order to estimate the originally transmitted data, i.e. before symbol detection takes place, the receiver has to cancel out interferences as far as possible. Besides noise, e.g. Gaussian distributed white noise, the received signals are distorted by inter-antenna interferences, abbreviated IAI, and inter-symbol interferences, abbreviated ISI. Consequently, i.e. in order to improve estimation of the originally transmitted data, both the IAI and the ISI should be cancelled out for each receiving antenna.
Besides high throughput, energy efficiency of mobile devices is of the utmost importance due to limited battery life time. Therefore, modulation and multiple-access schemes like single-carrier (SC) frequency-domain multiple-access (FDMA) are applied to the uplink, ensuring that the peak-to-average power ratio (PAPR) of the transmit power amplifiers is low. Such an uplink scenario is shown in FIG. 1 as described in more detail below. However, the main drawback of the SC-FDMA scheme is the presence of additional ISI, caused by multipath propagation. The more subcarriers of the available frequency band are allocated, the more ISI corrupts the received data streams. Therefore, especially wideband signals are prone to significant ISI that may dominate the overall interference.
Inter-symbol interference may be caused by non-orthogonal transmit filtering by the channel or by multipath propagation leading to an inherent non-linear frequency response of the transmission channel. In a multipath propagation environment a radiated signal may take different propagation paths, i.e. paths of different length, before arriving at a receiver antenna, where the signals may superpose or heterodyne, thus the signals do not arrive at the same time. In case the transmission channel exhibits a non-linear frequency response, a portion of the frequencies of a modulated signal is removed by the channel thus having impact on the pulse form of the received signal and causing successive symbols to blur together.
The phenomenon of inter-antenna interference is caused by using more than one antenna for transmitting on the same frequency resource, i.e. the two transmit antennas radiate at the same time using the same frequency, and thus the radiated signals interfere with each other.
Conventional techniques for interference reduction in MIMO receivers are linear filter operations, imposing low detection performance. As an example, the well-known MMSE detector multiplies a received signal vector by a fixed filter matrix, which is based on the estimated MIMO channel. Linear receiver solutions generally imply significant performance loss and therefore require high signal-to-interference-and-noise ratio conditions, abbreviated SINR, for reliable detection of the data streams. SINR can be increased by increasing the transmit signal power, which is hardly acceptable for mobile devices.
Enhanced but still low detection performance can be accomplished by stepwise interference reduction methods, i.e. the estimated interference of already detected data is subtracted from subsequently transmitted, not yet detected data. When considering IAI reduction, a famous method is the successive interference cancellation, abbreviated SIC. A similar method for ISI reduction is the application of decision feedback equalization, abbreviated DFE, which can be carried out either in time or in frequency domain.
So far, these methods can be applied either to wideband single-antenna systems, narrowband MIMO systems or wideband MIMO systems. Those equalization/detection methods are applied to the total interference and do not take the different interference characteristics into account. A two-stage approach as known from recent research separates the task into equalization and detection to mitigate ISI and TAT, respectively. The ISI equalization is carried out using a linear and fixed MMSE filter while the MIMO detection is carried out e.g. by a non-linear tree-search. Those tree-search-based approaches e.g. sphere detectors, that search for the most likely symbol within a given solution space, outperform MMSE and SIC significantly but come with increased computational complexity costs. Nevertheless, when implemented efficiently in hardware, sphere detection can be a promising detection method even for 4G systems that typically require high throughputs.
Besides specific equalization and detection processing, it is always possible to apply channel coding schemes in order to increase the robustness of the transmission. Furthermore, by means of iterative information exchange between detector and decoder, an additional significant detection gain (depending on the number of iterations) can be achieved. This so-called turbo principle is similarly used in modern decoder units, for example in low density parity check (LDPC) decoders.
Although immense research effort on efficient tree-search-based MIMO detection led to improved performance-complexity trade-off, the impact of frequency-selective channels is a problem. On the one hand, as soon as significant ISI is present in the received signal, the performance gain of two-stage approaches compared to low-complex linear detectors reduces dramatically. On the other hand, including ISI equalization in the sophisticated MIMO detection process would lead to an impractical overall complexity.
Hence, a wideband MIMO system based on single-carrier transmission schemes is required that efficiently tackles inter-symbol interference (ISI) and inter-antenna interference (IAI) thus enabling reliable symbol estimation.